The invention relates to improvements in papermaking machines, and more particularly to a method and mechanism for positive web transfer and control without rewetting in the press section of a paper machine.
In the transfer of a web between presses or from a press to a dryer, different structures and method have been employed. One arrangement is to carry the web across an open draw. However, since the sheet is unsupported, the open draw often limits the maximum machine speed due to the strength of the sheet. With types of webs made of short fibers or pulp such as bagasse, and with constructions wherein the sheet is unusually wet and heavy, breakage can occur much too easily in an open draw due to sheet flutter or normal stresses so that the use of open draws is limited to lower speeds and certain types of webs.
Another form of sheet transfer is with the sheet carried on a felt. This arrangement eliminates the problems of sheet breaks at an open draw, but there is a disadvantage in that a considerable amount of rewetting of the sheet from the felt occurs. The rewetting is counter-productive inasmuch as the function of the press is to remove as much moisture as possible to reduce the thermal energy expenditure necessary in the paper machine dryer section.
Another form of sheet transfer is on a press roll and although this eliminates the disadvantages of both the open draw and the rewetting of the sheet in felt transfer, there are geometric problems involved in having enough space around the roll to install all the associated press equipment for such transfer. The location and transfer space is determined by the location of the surface of the press roll.
It is accordingly an object of the present invention to provide an improved method and structure for the transfer of a web between presses or from a press to a dryer in a papermaking machine which eliminates the disadvantages accompanying open draw transfer, felt transfer or direct roll transfer.
A further object of the invention is to provide an improved web transfer arrangement in which the web is under complete control to avoid breakage, wherein rewetting from a felt is eliminated, and space problems are not limiting as to where the transfer is to take place.
A feature of the invention is to provide press arrangements which can either be the typical roll couple press or what has become known as extended nip press wherein prior to the transfer, the web is sandwiched between a felt and a looped endless belt which is smooth and relatively hard so that no rewetting of the sheet occurs on the offrunning side of the press nip and the sheet follows the belt after the press nip to be subsequently removed therefrom for the transfer. This arrangement utilizes the phenomena known to papermakers that the web will follow the surface having the greatest density and will follow a smooth impervious surface rather than a felt when the two are separated.
Other objects, advantages and features, as well as equivalent structures and methods which are intended to be covered herein will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings, in which: